CAUTION: The following post (or a page linked to) includes or discusses loading data not covered by currently published sources of tested data for this cartridge (QuickLOAD or Gordon's Reloading Tool data is not professionally tested). USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assumes any liability for any damage or injury resulting from the use of this information.
I've heard of the 45-70 being loaded to 60 KPSI, but only in the Ruger single-shot actions. The Marlin 1895 has most folks recommending not over 40 KPSI, but M.L. McPherson has driven it into the 50s. Logic suggests that if it is happy with a constant diet of 40 KPSI ammo, then 60 KPSI won't exceed the upper proof range but would certainly cause me to keep my eyes peeled for any conventional pressure sign, and I would run a micrometer up and down the barrel initially so that if the steel start showing signs of assuming a permanent increase in diameter, I could back the loads off. In the 19th century, when proof testing canons, the British naval gun makers had to demonstrate that the traditional double-load test did not cause more than 0.1" change in OD. This will be right around 0.001" in the OP's gun size and would be a good check to make starting at the breech and going forward every inch and getting a record to see that it doesn't change.
1972RedNeck,
A couple of more items: Do you have the 22", 24", or 26" barrel?
Another item: since you are using Starline brass (good stuff) rather than antique spec brass, we need an internal volume measurement of an as-fired Starline case. The way you do this is to weigh your fired cases, pick an average value, and record it. Preferably, it should have been fired with one of your existing higher-pressure loads. This will cause the internal volume of the case to reflect your chamber's actual capacity best. Then, measure and record that sample case's as-fired length. Then, fill it with room temperature water level with the case mouth (no meniscus, positive or negative) and weigh it again. The difference between the empty weight and the full weight is called case water overflow capacity, and that is needed in QuickLOAD as one of the parameters.
When filling the case with water, tap it periodically to float bubbles out. If you overfill it, twist the corner of a Kleenex into a little rat tail, touch it to the positive meniscus briefly, and repeat as needed to get the water level flat at the case mouth. Also, dry off any water drops on the outside of the case before weighing.
Put the information here.
I can tell you that with QuickLOAD's default case water overflow capacity for this cartridge (88 grains), filling to 100% loading density with 2015 produces 38,691 psi and 2005 fps from a 24" tube. Reloader 7 at 100% produces 56,033 psi and 2158 fps. Your 67-grain load of 3031 fills the default case 126.5%, which is too much compression to avoid bulging a case, and even if you could squeeze it in, the pressure is predicted to be north of 70,424 psi, and the velocity at 2409 fps from the 24" tube. Given the impossible compression, I expect your case water overflow capacity to be higher than the default. It is common for QuickLOAD default case capacities to be conservatively small to discourage overloading.